Improve terminal output backpressure

Adds ACK-backed renderer output admission control for PTY data, bounding per-PTY and global renderer in-flight output while preserving PTY ingestion for runtime state and notifications.

src/preload/api-types.ts

@@ -901,6 +901,7 @@ export type PreloadApi = {
     signal: (id: string, signal: string) => void
     kill: (id: string, opts?: { keepHistory?: boolean }) => Promise<void>
     ackColdRestore: (id: string) => void
+    ackData: (id: string, charCount: number) => void
     hasChildProcesses: (id: string) => Promise<boolean>
     getForegroundProcess: (id: string) => Promise<string | null>
     getCwd: (id: string) => Promise<string>

src/preload/index.ts

@@ -683,6 +683,9 @@ const api = {
     ackColdRestore: (id: string): void => {
       ipcRenderer.send('pty:ackColdRestore', { id })
     },
+    ackData: (id: string, charCount: number): void => {
+      ipcRenderer.send('pty:ackData', { id, charCount })
+    },
 
     kill: (id: string, opts?: { keepHistory?: boolean }): Promise<void> =>
       ipcRenderer.invoke('pty:kill', { id, keepHistory: opts?.keepHistory ?? false }),